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Morphology and composition of Au catalysts on Ge(111)
obtained by thermal dewetting
S Hajjar, G Garreau, L Josien, Jean-Luc Bubendorff, D Berling, A Mehdaoui,
C Pirri, T Maroutian, C Renard, D Bouchier, et al.
To cite this version:
Morphology and composition of Au catalysts on Ge(111) obtained by thermal dewetting
S. Hajjar, G. Garreau, L. Josien, J. L. Bubendorff, D. Berling, A. Mehdaoui, C. Pirri*.
IS2M, Université de Haute Alsace, CNRS-LRC 7228, 68057 Mulhouse, France
T. Maroutian, C. Renard, D. Bouchier
IEF, Université Paris-Sud, UMR 8622, Orsay, F-91405 and CNRS, Orsay, F-91405
M. Petit, A. Spiesser, M. T. Dau, L. Michez, V. Le Thanh
CINaM-CNRS, Aix-Marseille Université, Campus de Luminy, Case 913, 13288 Marseille Cedex 9
T. O., Mentes, M. A. Nino, A. Locatelli
Sincrotrone Trieste, Area Science Park, Trieste 34012, Italy
PACS numbers: 73.63 Kv, 68.37 Ef, 64.75.Jk, 61.46.-w
Abstract:
We investigate the chemical and morphological structure of the Au nanodots on Ge(111)
which serve as catalysts for the formation of epitaxial Ge nanowires. The spatial localization
of Au is investigated by X-ray spectromicroscopy and transmission electron microscopy. We
show that dewetting of an Au film on Ge(111) gives rise to a thin Au-Ge wetting layer and
Au-Ge dots. These dots are crystallized but not with a single crystallographic orientation.
Thanks to the spatially resolved X-ray and transmission electron microscopy measurements, a
chemical characterization of both binary Au-Ge catalysts and wetting layer is obtained at the
I. Introduction:
The growth of nanowires by the vapor-liquid-solid (VLS) or vapor-solid-solid (VSS)
mechanism is a well-known and very nice bottom-up approach in the fabrication of
one-dimensional objects, which can be used in a very large variety of devices 1-5. The
experimental VLS approach needs alloys that can form nanoscale catalysts, which melt at low
temperatures thanks to the deep eutectic point in the bulk phase diagram. Most interesting is
that these alloys are also of interest in the overall microelectronic area as solder materials and
in all technological area in which low temperature and corrosion resistance are required, such
as space technology, gas sensor and medical devices. The nanowires geometry is ideal for
monolithic integration of semiconductor materials with different lattice constants due to their
ability to accommodate strain in two dimensions. As to nanowires growth, if an epitaxial
growth is required to perform devices, these catalysts are formed under ultrahigh vacuum on a
clean and crystalline substrate. In some case, their chemical and morphological properties
have been investigated. Nevertheless, the chemical reaction at the interface between the
deposited material, which serves to form the catalysts, and the semiconductor surface strongly
depends on their chemical nature. This point was extensively and is still studied in the
formation of thin layers (two-dimensional) interfaces but it becomes much more complex for
surfaces on which dots are formed. On one hand it needs particular tools for local
measurements and on the other hand, the reduced size of the catalysts and of the wires grown
through it highlights new phenomena. Numerous materials have been tentatively used as
catalysts, each with its own influence on the nanowires growth, such as nanowires crystal
orientation and growth orientation with respect to the semiconductor surface. Among these
catalysts, Au droplets have been extensively used5-42and a particular attention is given at the
below 1 monolayer has thoroughly been investigated by photoemission, Auger electron
spectroscopy, LEED and X-ray diffraction 43-50. It is characterized by the formation of a √3 x √3 R30° superstructure (or wetting layer) associated with the formation of Au trimers on Ge(111)48-50. It is found to be stable at high annealing temperatures, up to the melting point of
germanium 45. In contrast, the Au/Ge(111) interface has only poorly been studied for Au
deposits above 1 monolayer. It is worth noting that Au catalysts are generally created by
dewetting a pure Au film of about 1 nm thick and up to now only few papers report on the
characterization of the Au droplets before nanowires growth.
In this work, we investigate both chemical and morphological structure of the Au
platelets on Ge(111), formed by annealing a pure Au film at a temperature below 300°C, and
of Au droplets, formed at higher annealing temperature, which serve as catalysts for the
formation of epitaxial Ge nanowires. This study is performed by using Scanning Tunneling
Microscopy (STM), X-ray Photoemission Spectroscopy (XPS) and X-ray Photo-Diffraction
(XPD), Reflection High Energy Electron Diffraction (RHEED), and finally by using X-ray
Photoemission Electron Microscopy (X-PEEM) and Transmission Electron Microscopy
(TEM) in both image and microanalysis modes.
II. Experiments.
The sample preparation, as well as the STM, RHEED and X-rays photoelectron
diffraction (XPD) measurements, were performed in UHV setup with a base pressure below 1
x 10-10 mbar. STM measurements were made in a room-temperature-operating microscope
(Omicron STM-AFM microscope), in the constant-current mode. Electrochemically etched, in
situ cleaned tungsten tips were used. The Au catalysts were grown on a clean Ge(111)
and flashed afterwards at 720 °C to remove the native oxide layer. After repeated flashes at
720 °C for increasing durations (up to one minute), the substrate was cooled rapidly down to
860 °C and then more slowly (at a rate of 0.5 °C/s) down to RT. STM images taken on a clean
Ge(111) substrate show terraces larger than 200 nm and a quite defect-free c(2x8) surface
atomic structure. Au catalysts were formed by annealing Au layers evaporated on such a
Ge(111) substrate kept at room temperature. An effusion cell was used with a deposition rate
of about 0.05 nm / min. The Au layer thickness was set between 0.6 and 1.2 nm. This Au
amount gave us the opportunity to form Au-Ge droplets with a lateral size between 5 and 200
nm, then easily observable by STM for the smallest one, and by SEM and X-ray
spectromicroscopy for the largest. The deposition rate is controlled by a water-cooled quartz
crystal microbalance and the nominal Au thickness is given with a precision better than 10%.
The annealing temperature is monitored with an accuracy of ± 20°C. XPD measurements
were carried out using a hemispherical analyzer operating at an angular resolution of ±1°
(Omicron experimental set-up). XPD scans were obtained by measuring the intensity of the
Au 4f core level doublet excited with an Al Kα x-ray source (photon energy =1486.6 eV).
The local X-ray spectromicroscopy measurements are performed with the XPEEM–LEEM
microscope at Elettra laboratory in Trieste (Italy) on the Nanospectroscopy beamline, which
routinely works with spatial resolution of 40 nm in XPEEM mode. Details on the microscope
and the beamline are reported in ref.51, 52. Using synchrotron light as a photon source we
were able to select the photon energy for probing both Ge3d and Au4f core level emission
under optimal conditions. TEM and EDX were performed with a JEOL 3010 microscope
operating at 300 keV with a spatial resolution of about 2 nm.
Figure 1a shows an image of a 0.8 nm thick Au layer deposited at room temperature
on Ge(111). The Au film is rather flat and covers quite uniformly the Ge surface. LEED and
RHEED pattern is 1x1 and shows that Au is ordered (in epitaxy) on Ge(111), in agreement
with previous findings 45,46. XPD Au4f7/2 line intensity scans versus polar angle θ along the
[11-2], [-1-12] and [10-1] directions of the Ge(111) crystal are shown in Figure 1b. The polar angle θ is defined with respect to the surface normal. These intensity modulations versus polar angle show intensity maxima at selected polar angles, consistent with the formation of a
face-centered cubic Au structure, with Au(111) // Ge(111), and with [11-2], [-1-12] and [10-1]
directions of Au aligned with that of Ge(111), as observed in ref. 45.
The Au layer is scattered into small islands upon a mild annealing at 300 °C, as shown
in Figure 2. This is the first stage of dewetting process. The STM image shows numerous flat
platelets with different lateral size and height. Their height varies from 2.5 to 4.5 nm for a
nominal deposit of 0.8 nm and an annealing at 300°C for 10 min. A line scan across two
islands is shown in Figure 2. As to their structure, Figure 2 also shows a XPD Au 4f7/2profile
(a) versus polar angle θ along the [11-2] direction of the Ge(111) substrate. It is compared to
that measured on the room-temperature deposited Au layer (b). This close similarity shows
that the Au platelets are still in epitaxy and single-oriented on the Ge(111) substrate. At this
stage, an estimation of the Au platelet volume, with respect to the initial Au deposit, suggests
that they are quite pure Au. Thus, we may assume here that the bare surface does not
significantly contribute to the Au4f modulations versus polar angle. The relevance of this
point will be discussed later.
Figure 3 shows a typical STM image acquired after a subsequent anneal at 325°C for
10 min. This extra anneal does not change the overall surface morphology. Nevertheless, a
close examination shows that several islands have changed their shape. A line scan across
respectively). The line scan shows that the dome island height is at least twice that of the flat
island. The morphology change observed at this temperature can be attributed to the
formation of Au-Ge droplets at the eutectic composition and thus Ge incorporation. From
bulk Au-Ge phase diagram, the droplet could incorporate 28 at% Ge at the eutectic
temperature TE. This would increase the dome-like islands volume of about 33% if we assume
that the bulk phase diagram predictions are still correct for the small droplets and if the Ge
incorporated at TE remains within the islands at room-temperature. This latter point strongly
depends on the time used to decrease the sample temperature from 325°C down to
room-temperature. In the present experiments, this time was rather short (less than 5 minutes) and
we can assume that the droplet composition is quenched or partially quenched. This point will
be also discussed later on the basis of SEM experiments. Upon increasing the annealing
temperature up to 350°C, the surface morphology has completely changed, as evidenced in
Figure 4. This STM image is acquired after annealing the surface at 350°C for 10 minutes.
There is not a track anymore of platelets. All islands have now a dome shape. Upon
increasing the annealing time at 350°C, Ostwald ripening process induced a modification of
the sample surface: the islands density is reduced upon increasing annealing time, while their
height and diameter increases. In particular, it is shown that the mean droplet diameter
increases continuously, quite linearly, versus annealing time, even for durations as long as 12
hours. Figure 5 gives an STM image of the surface after 12 hours annealing at 350°C. Au
droplets as large as 100 nm are now formed. These droplets are crystallized and show facets.
At a first sight, all droplets do not show facets in the same crystallographic direction, as it
would be expected for Au island in epitaxy on Ge(111). A detail view of a droplet is shown in
Figure 5. Note that the ripening process is a limiting factor in the fabrication of nanowires
with low size dispersion. Despite the well defined crystallographic phases, the Au4f7/2
more coherence between the droplets orientation after melt. Such a profile indicates the
formation of several nanocrystal orientations, as it is for a polycrystalline surface.
To summarize, an overview of the evolution of the surface morphology versus
annealing temperature at a given annealing time, versus annealing time at a given temperature
and versus Au layer thickness in a range generally used for nanowires growth is shown in
Figure 6. Figure 6A shows a set of STM images taken in the constant current mode for
different Au layer thickness annealed at 350°C (TE). Figures 6B, 6C and 6D show the mean
droplet diameter, the mean droplet height and the droplet density versus annealing duration at
350°C for an Au deposit of 0.8 nm, respectively,. It is shown that the droplet density
decreases as the mean droplet diameter and height increases, versus annealing time. In
particular, it was shown that the mean droplet diameter increases continuously, quite linearly,
versus annealing time, even for durations as long as 12 hours.
Some comment must be given about the in-situ determination of the solid-liquid
transition temperature of the droplets. We have chosen to estimate the transition temperature
TE by using the change on STM images acquired at room temperature. We have determined
the temperature at which the platelets transform into dome shape islands (droplets) and
assumed that it is TE, indeed. This temperature is estimated for annealing time longer than 1
hour. As shown below, RHEED can also confirm the structural transition from crystallized
platelets (2D crystals) to liquid droplets (3D liquid). RHEED measurement gives
complementary and valuable information on the droplets structure. Starting from a clean
Ge(111) surface characteristic by a c(2x8) or (2x1) pattern, a (1x1) streaky RHEED pattern is
observed for room-temperature Au deposition up to a thickness of 1.2 nm. This indicates that
the deposited Au film is relatively flat and epitaxial, in agreement with STM and XPD results.
Figure 7A displays a RHEED pattern taken along the [1-10] azimuth of a 1.2 nm thick Au
as-deposited pattern, we observe here the appearance of three-dimensional (3D) spots, which can
be attributed to the transmission diffraction effect across Au platelets as observed in Fig. 3A.
The 3D spots are arranged in a pseudo-hexagonal symmetry and the fact the all these spots are
located along the (1x1) streaks confirms that these platelets are coherent and epitaxial. We
note that (1x1) streaks are still present, indicating that the Au wetting layer in between
platelets remains flat. When annealing at 350°C (Fig. 7B), 3D spots are still present but
interestingly they are distributed along concentric rings, a behavior similar to that observed
from electron diffraction of a polycrystalline structure 53-58. This is in line with STM
measurement, for which it is although difficult to have a good statistics over all orientation of
droplets. This is also in good agreement with the above XPD analyses depicted in the curve c
of Figure 5, which suggests that, when Au droplets are formed for annealing at temperatures
higher than TE, they are no longer epitaxial but exhibit a random distribution. In other words,
upon Ge incorporation the Au-Ge droplets are no longer made up of (111) planes parallel to
the (111) plane of Ge substrate but are randomly oriented.
An important question here is: is there still Au between the platelets and between the
droplets and how many Au? If it is so, this could also participate to the Au4f7/2 modulation
versus polar angle. This point has been addressed in the literature a long time ago. Indeed, the
formation of the Au/Ge(111) interface has been extensively studied in the 0-1 Au monolayer
range, in the two past decades. It has been shown that a √3 x √3 R30°-Au superstructure
appears upon annealing a monolayer Au deposit above 300°C. This superstructure was found
to be stable up to the melting point of Ge (958.5°C). Several atomic models have been
proposed for this superstructure. For all models, this superstructure consists of a surface on
which Au atoms replace the topmost Ge atoms of the substrate and form trimers. Au atoms
replace either the outermost Ge layer or the second Ge layer. This could be in line with the
formation is generally out of thermodynamically equilibrium, with the formation of new and
metastable phases. Anyway, the nominal amount of Au involved in this reconstruction is of
one Au monolayer. It was also found that this superstructure induces strong distortion in the
deeper Ge layers, with most notably a buckling in the third and fourth Ge layers 50. For very
low coverage, the surface periodicity is more complicated since it was observed a “split”
(2x2) periodicity along with the √3 x √3 R30° superstructure45. For both Au positions on the
Ge(111) surface proposed in the literature, the XPS Au4f wave is not expected to experience
forward scattering and its contribution would not be detected at polar angles below 60° in
XPS profiles. Some small contribution, as an increase of the mean intensity, would be
detected at large polar angles, as it is shown for two-dimensional layers 63, 64. Thus, it can be
safely assumed that the “bare surface” does not significantly contribute to the Au4f XPD
profiles. The Au4f XPD profiles in Figures 2 and 5 are clearly representative of the Au
droplets or platelets, only. The regime of higher Au coverage (more than 1 Au monolayer) has
not been so extensively studied, the focus being on the √3 x √3 R30° itself, due to the
universality of its occurrence for the metal/semiconductors interfaces.
Figure 8 shows a SEM image collected at room temperature for a 1.2 nm Au deposit
after anneal at 350°C and 400°C for 1 hour. This deposit is slightly larger than that used for
STM, to be easily observed by SEM. STM shows that increasing the deposit from 0.8 to 1.2
nm does not significantly change the surface morphology. Figure 8 shows that each droplet is
crystallized, as shown by STM. Also shown in Figure 8 is the dispersion in the crystallized
droplets shape, in line with randomly oriented nanocrystals, as suggested by RHEED and
STM. Furthermore, one can see that the largest droplets are perched on a pedestal after
annealing at 350°C and 400°C. This pedestal could be due to precipitated Ge (or Au-Ge
alloy), suggesting that quenching of the droplet composition is not completely efficient. The
liquid droplet form. Similar pedestal has already been observed for Au seeds melt on Si(111).
For the Au/Si(111), these pedestals were also attributed to Si precipitation and thus Au and Si
phase separation on the basis of selective etching experiments 65-67. However, the nice
experiments reported in ref. 65, 66 do not clarify the crucial point of Si-Au alloy formation
and composition. A chemical information has thus to be probed by using spectroscopic
investigations at a nanometer scale.
Chemical information on Au or Au-Ge nanocrystals is gained by using XPEEM and
TEM experiments. These techniques are used in both image and spectroscopy modes. EDS
spectroscopy used in TEM with a focalized spot allows a good spatial resolution in
cross-section images. Nevertheless, due to the large depth probed by the electrons, it is less suitable
for a chemical analysis in the in-plane mode. The analysis of the lateral distribution of Au is
more convenient by using a tool as nanospectromicroscopy XPEEM.
Figure 9A shows a PEEM image of the sample surface for 1.2 nm Au deposit annealed
at 350 °C for 12 hours. These experimental conditions are chosen to have droplets large
enough to be analyzed since the spatial resolution is about 40 nm in the XPEEM imaging.
This image was acquired in the X-ray absorption mode, at the Ge3d edge. The field of view of
this image is 4μm. Due to the presence of Ge overall sample surface and due to the large
depth probed at this photon energy, all parts of the sample appear with the same grey scale.
Nevertheless, thanks to the high photon beam angle, the Au islands are visualized, without
significant chemical contrast. The photon beam comes from the bottom-left, with an
illumination angle of 74° with respect to the surface sample. This image shows Au dots with a
diameter in the 150 - 200 nm range. The topographic contrast is also enhanced by a strong
emission of secondary electrons on the bottom-left side of the islands.
The presence of Au atoms between the islands is confirmed by XPEEM. The analysis
imaging. Figure 9A shows a XPEEM image in the XPS mode. In order to maximize surface
sensitivity, the photon energy was set to 201 eV, corresponding to a kinetic energy of 113 eV
of the Au 4f electrons, which is close to the minimum of the inelastic mean free path for this
material. The droplets are visualized thanks to the illumination angle and to a slight difference
in the Au4f intensity between surface and dots. Figure 9B shows the Au4f line measured on
both droplets and surface in between. These spectra show normalized Au4f7/2 intensity versus
binding energy. The Au 4f binding energy EB measured on droplets is almost the same as for
bulk Au at EB = 84.0 ± 0.2 eV 49, 68-71. The binding energy is measured by also scanning
across the Fermi level EF. This suggests that the droplet surface is almost pure Au. The Au4f
lines are shifted each other by about 0.45 eV. This chemical shift is consistent with that
observed for Au4f measured on the Ge(111)-Au√3x√3 R30° surface and on bulk gold49. The
droplets, with a mean diameter of 150 nm, are large enough to neglect any final state effect on
the Au4f binding energy 68-71. Note that a straightforward determination of the Au to Ge
composition cannot be safely extracted from XPEEM data and it would be only a picture of
the droplets for a given growth condition. Indeed, it strongly depends not only on the
annealing temperature but also on the rate at which the temperature goes down when the
sample returns at room temperature. It will be shown below that a strong composition gradient
occurs along the surface normal since Ge precipitation is a diffusion limited process.
Cross-section TEM images give us more information about the droplet lateral size, the alloyed zone
at the Au/Ge(111) interface and the alloy extension in the droplet and beneath.
Figure 10 shows TEM images for a 1.2 nm Au deposit annealed at 350°C (A), (B) and
400°C (C). The sample temperature was decreased down to room temperature at of rate
higher than 10°C/min. These images show Au-Ge droplets of about 50-100 nm lateral size,
thus in the range used for XPEEM. It is shown that the dots height, with respect to the surface
annealed at low temperature rest on a 1 nm thick wetting layer with a quite uniform thickness.
In contrast, the dots annealed at higher temperature penetrate more deeply in the Ge(111)
substrate. The wetting layer is still observed, with almost the same thickness, but the droplets
extent well below the Ge(111)/wetting layer interface. At a first sight, this shows a strong
pinning of the droplets when the annealing temperature is increased above TE. Such
self-pinning effect has also been suggested for the Au/Si(111) system by Ferralis et al. 66. The
present measurements give a direct proof of this effect. Chemical information is given by
EDX measurements performed on both droplets type and also on the wetting layer. Figures 10
D, E and F show the GeL, GeK and AuM lines intensity across the white line at selected
points, for the bare surface (wetting layer), the droplet annealed at 350°C and the droplet
annealed at 400°C, respectively. As to the wetting layer, a very small Au signal is detected, in
agreement with the formation of a diluted GeAu alloy. All models of the √3x√3 R30°
reconstruction include only one Au atomic layer on top of the Ge(111). Furthermore, the
strong interaction of that single Au layer with the substrate induces strong distortion of the Ge
network underneath, as shown by H. Over et al. by using dynamic LEED measurements 50.
These authors proposed Ge displacements up to the sixth atomic plane in the substrate. This
could be at the origin of the contrast observed by TEM. Nevertheless, the extent of the
wetting layer observed by TEM seems too large compared to that proposed in refs 48-50. A
part of the wetting layer observed by TEM could also be associated with in-plane Ge
precipitation. This point deserves further investigation.
As to the droplets, Figure 10E and 10F show Ge and Au signals on droplets annealed
at 350°C, i.e. close to the eutectic temperature TE, and at 400°C. Figure 10E shows that Au
signal is clearly sizeable for two points only, namely points 3 and 4, thus in the droplet and
above the wetting layer. In line with that, Ge signal is at his maximum for points 1 and 2, in
Au content is almost the same at the droplet base and on the top of it. One can assume that the
Au to Ge composition is quite uniform in the droplet upon annealing at TE. In contrast, Figure
10F shows that the Au signal strongly depends on position in the droplet. Indeed, a large Au
signal is only measured at points 7 and 8 although the droplet extends from point 4 to point 8.
For points 5 and 6, a smaller Au signal is measured, larger that that at points 1 and 2, which
are located in the substrate. These measurements clearly show that the vertical growth of the
droplet is associated with a severe Au redistribution in it. The EDX profiles suggest that the
Ge to Au composition is quite the same at the points at which AuM line is at maximum. At
higher annealing temperature, a larger amount of Ge is incorporated in the droplet, which
increases in height. From bulk phase diagram, the alloy composition changes from Au78Ge28
to Au74Ge32 when the annealing temperature is increases from 350°C to 400°C. Ge
incorporated during the annealing process is now precipitated along the surface plane, to give
pedestal and/or wetting layer (this depends on the point of view, in-plane or cross-section) but
also serves to increase the Au droplet height, as show in Figures 10F. Nevertheless, the
composition variation from Au78Ge28 to Au74Ge32upon increasing the temperature at 400°C
seems too low to have such effect on the droplet shape and Ge distribution in it. This would
only explain a height increase of about 4%, i.e. far from that observed in Figure 10. This
seems also suggested by the SEM tilted image shown in Figure 8. The driving force for the
longitudinal droplet growth is then Ge incorporation process during temperature increase, thus
climbing the phase diagram liquidus/solidus (L-S) line, and Ge precipitation during
temperature decrease, thus going down the L-S line. Nevertheless, this process alone cannot
explain the large change in volume occupied by this extra Ge. Surface energy driven
agglomeration has to be considered. The bulk phase diagram is modified upon decreasing the
droplet size. Experimental evidence has been reported at the Au-Ge / Ge interface on top of a
important change in the Ge at% is observed. This amounts at about 47 % at 450°C instead of
32% for bulk material for a Ge nanowire diameter of 32 nm. Nevertheless, this effect is much
weaker for droplets with diameter of about 200 nm or more, as it is in the present work. Some
important difference has although to be considered: in the present work, the droplet is in
contact with a two-dimensional surface instead of a wire in ref.28. The present experiments
suggest that this phenomenon could be enhanced on a two-dimensional surface. This opens
the way to large surface modification, via significant material transport across the surface by
using the peculiar properties of the binary or ternary alloys with a deep eutectic point in the
bulk phase diagram.
IV) Conclusion
We have investigated the formation of Au-Ge seeds formed by Au layer dewetting on
Ge(111) clean surface. We have shown that they are crystallized after melt and cooling down
to room temperature. The Au platelets are in epitaxy on Ge(111) but epitaxy is lost after melt.
As expected from bulk phase diagram, Au seeds incorporate Ge which precipitates to form a
pedestal upon cooling down the sample at room temperature. The interesting feature here is
that the Ge precipitated amount is larger (at least twice) that expected from bulk phase
diagram opening the way to large surface modification, via significant material transport
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Figures captions:
Figure 1: (A) STM image of 0.8 nm Au deposited onto a room-temperature (RT) clean
Ge(111). (B) Au 4f7/2 XPD profiles versus polar angle θ along the [11-2], [10-1] and [-1-12]
direction of the Ge(111) substrate.
Figure 2: (A) STM images of 0.8 nm Au deposited onto a room-temperature (RT) clean
Ge(111) annealed for 10 min at 300°C. Also shown in this image is a line profile across
islands. (B) Au 4f7/2 XPD profile versus polar angle θ along the [11-2] direction of the
Ge(111) substrate measured on the Au deposit annealed at 300°C (a) and on the
room-temperature deposited Au layer (b).
Figure 3: STM images of 0.8 nm Au deposited onto a room-temperature (RT) clean Ge(111)
annealed for 10 min at 325°C. Also shown in this image is a line profile which clearly
distinguishes flat and dome-like islands.
Figure 4: STM images of 0.8 nm Au deposited onto a room-temperature (RT) clean Ge(111)
annealed for 10 min at 350°C. Also shown in this image is a line profile across a dome-like
island.
Figure 5: (A) STM image of 0.8 nm Au deposited after 12 hours annealing at 350°C and (B) a
detail of a droplet. (C) Au 4f7/2XPD profiles versus polar angle θ along the [11-2] direction of
the Ge(111) substrate measured on the room-temperature (RT) (a), on the Au deposit
Figure 6: A) STM images (2 μm x 2 μm) of an Au deposit of 0.4 nm, 0.8 nm and 1.2 nm
annealed at 350°C for 10 min. Also shown for a 0.8 nm Au deposit annealed at 350°C: B) the
droplets diameter versus the annealing time, C) the droplets height versus annealing time and
E) the proportion of the surface covered by droplets versus annealing time.
Figure 7: RHEED pattern measured on a 1.2 nm thick Au layer annealed at 350°C. The
primary energy is 30 keV and the angle of incidence is < 0.77° from the surface.
Figure 8: SEM images collected at room temperature for a 1.2 nm Au deposit after anneal at
350°C (A) and 400°C (B, C, D) for 1 hour. The topmost images (A) and (B) are 45° tilted
images to enhance the observation of the pedestal.
Figure 9:(A) XPEEM image of the sample surface for 1.2 nm Au deposit annealed at 350 °C
for 12 hours. This image acquired in the XPS mode. The sample is illuminated at photon
energy of 201 eV and the lateral distribution of the Au4f intensity is taken at a kinetic energy
of Ec= 113 eV, thus at the Au4f7/2line maximum. The field of view is 4μm. (B) Normalized
Au4f lines measured on droplets (a) and surface in between (b).
Figure 10: Cross-section TEM images for a 1.2 nm Au deposit annealed at 350°C and 400°C.
This figure also shows the GeL, GeK and AuMα lines intensity across the yellow line of the
Figure 3
Figure 4
Figure 5
Figure 7
